A newborn baby or child of any age.
A pet.
A child or animal that is born singly, not as a twin or other multiple birth.
A single consonant, as opposed to a geminated consonant.
A single object, especially one of a group.
A playing card that is the only one of its suit in a hand, especially at bridge.
A person without a romantic partner.
A hand containing only one card of a certain suit.
A set with exactly one element.
A person without a dissociative identity.
A class that may not be instantiated more than once, i.e. that implements the singleton design pattern.